The present invention relates to coding apparatuses and methods in which a feature quantity obtained from an audio signal such as a voice signal or a music signal, especially a signal obtained by transforming an audio signal from time-domain to frequency-domain using a method like orthogonal transformation, is efficiently coded so that it is expressed with fewer coded streams as compared with the original audio signal, and to decoding apparatuses and methods having a structure capable of decoding a high-quality and broad-band audio signal using all or only a portion of the coded streams which are coded signals.
Various methods for efficiently coding and decoding audio signals have been proposed. Especially for an audio signal having a frequency band exceeding 20 kHz such as a music signal, an MPEG audio method has been proposed in recent years. In the coding method represented by the MPEG method, a digital audio signal on the time axis is transformed to data on the frequency axis using orthogonal transform such as cosine transform, and data on the frequency axis are coded from auditively important data by using the auditive sensitivity characteristic of human beings, whereas auditively unimportant data and redundant data are not coded. In order to express an audio signal with a data quantity considerably smaller than the data quantity of the original digital signal, there is a coding method using a vector quantization method, such as TC-WVQ. The MPEG audio and the TC-WVQ are described in xe2x80x9cISO/IEC standard IS-11172-3xe2x80x9d and xe2x80x9cT. Moriya, H. Suga: An 8 Kbits transform coder for noisy channels, Proc. ICASSP 89, pp. 196-199xe2x80x9d, respectively. Hereinafter, the structure of a conventional audio coding apparatus will be explained using FIG. 37. In FIG. 37, reference numeral 1601 denotes an FFT unit which frequency-transforms an input signal, 1602 denotes an adaptive bit allocation calculating unit which codes a specific band of the frequency-transformed input signal, 1603 denotes a sub-band division unit which divides the input signal into plural bands, 1604 denotes a scale factor normalization unit which normalizes the plural band components, and 1605 denotes a scalar quantization unit.
A description is given of the operation. An input signal is input to the FFT unit 1601 and the sub-band division unit 1603. In the FFT unit 1601, the input signal is subjected to frequency transformation, and input to the adaptive bit allocation unit 1602; In the adaptive bit allocation unit 1602, how much data quantity is to be given to a specific band component is calculated on the basis of the minimum audible limit, which is defined according to the auditive characteristic of human beings and the masking characteristic, and the data quantity allocation for each band is coded as an index.
On the other hand, in the sub-band division unit 1603, the input signal is divided into, for example, 32 bands, to be output. In the scale factor normalization unit 1604, for each band component obtained in the sub-band division unit 1603, normalization is carried out with a representative value. The normalized value is quantized as an index. In the scalar quantization unit 1605, on the basis of the bit allocation calculated by the adaptive bit allocation calculating unit 1602, the output from the scale factor normalization unit 1604 is scalar-quantized, and the quantized value is coded as an index.
Meanwhile, various methods of efficiently coding an acoustic signal have been proposed. Especially in recent years, a signal having a frequency band of about 20 kHz, such as a music signal, is coded using the MPEG audio method or the like. In the methods represented by the MPEG method, a digital audio signal on the time axis is transformed to the frequency axis using orthogonal transform, and data on the frequency axis are given data quantities, with a priority to auditively important one, while considering the auditive sensitivity characteristic of human beings. In order to express a signal having a data quantity considerably smaller than the data quantity of the original digital signal, employed is a coding method using a vector quantization method, such as TCWVQ (Transform Coding for Weighted Vector Quantization). The MPEG audio and the TCWVQ are described in xe2x80x9cISO/IEC standard IS-11172-3xe2x80x9d and xe2x80x9cT. Moriya, H. Suga: An 8 Kbits transform coder for noisy channels, Proc. ICASSP 89, pp. 196-199xe2x80x9d, respectively.
In the conventional audio signal coding apparatus constructed as described above, it is general that the MPEG audio method is used so that coding is carried out with a data quantity of 64000 bits/sec for each channel. With a data quantity smaller than this, the reproducible frequency band width and the subjective quality of decoded audio signal are sometimes degraded considerably. The reason is as follows. As in the example shown in FIG. 37, the coded data are roughly divided into three main parts, i.e., the bit allocation, the band representative value, and the quantized value. So, when the compression ratio is high, a sufficient data quantity is not allocated to the quantized value. Further, in the conventional audio signal coding apparatus, it is general that a coder and a decoder are constructed with the data quantity to be coded and the data quantity to be decoded being equal to each other. For example, in a method where a data quantity of 128000 bits/sec is coded, a data quantity of 128000 bits is decoded in the decoder.
However, in the conventional audio signal coding and decoding apparatuses, coding and decoding must be carried out with a fixed data quantity to obtain a good sound quality and, therefore, it is impossible to obtain a high-quality sound at a high compression ratio.
The present invention is made to solve the above-mentioned problems and has for its object to provide audio signal coding and decoding apparatuses, and audio signal coding and decoding methods, in which a high quality and a broad reproduction frequency band are obtained even when coding and decoding are carried out with a small data quantity and, further, the data quantity in the coding and decoding can be variable, not fixed.
Furthermore, in the conventional audio signal coding apparatus, quantization is carried out by outputting a code index corresponding to a code that provides a minimum auditive distance between each code possessed by a code block and an audio feature vector. However, when the number of codes possessed by the code book is large, the calculation amount significantly increases when retrieving an optimum code. Further, when the data quantity possessed by the code book is large, a large quantity of memory is required when the coding apparatus is constructed by hardware, and this is uneconomical. Further, on the receiving end, retrieval and memory quantity corresponding to the code indices are required.
The present invention is made to solve the above-mentioned problems and has for its object to provide an audio signal coding apparatus that reduces the number of times of code retrieval, and efficiently quantizes an audio signal with a code book having a lower number of codes, and an audio signal decoding apparatus that can decode the audio signal.
An audio signal coding method according to the present invention is a method for coding a data quantity by vector quantization using a multiple-stage quantization method comprising a first-stage vector quantization process for vector-quantizing a frequency characteristic signal sequence which is obtained by frequency transformation of an input audio signal, and second-and-onward-stages of vector quantization processes for vector-quantizing a quantization error component in the previous-stage vector quantization process: wherein, among the multiple stages of quantization processes according to the multiple-stage quantization method, at least one vector quantization process performs vector quantization using, as weighting coefficients for quantization, weighting coefficients on frequency, calculated on the basis of the spectrum of the input audio signal and the auditive sensitivity characteristic showing the auditive nature of human beings.
Another audio signal method according to the present invention is a method for coding a data quantity by vector quantization using a multiple-stage quantization method comprising a first vector quantization process for vector-quantizing a frequency characteristic signal sequence which is obtained by frequency transformation of an input audio signal, and a second vector quantization process for vector-quantizing a quantization error component in the first vector quantization process: wherein, on the basis of the spectrum of the input audio signal and the auditive sensitivity characteristic showing the auditive nature of human beings, a frequency block having a high importance for quantization is selected from frequency blocks of the quantization error component in the first vector quantization process and, in the second vector quantization process, the quantization error component of the first quantization process is quantized with respect to the selected frequency block.
Another audio signal coding method according to the present invention is a method for coding a data quantity by vector quantization using a multiple-stage quantization method comprising a first-stage vector quantization process for vector-quantizing a frequency characteristic signal sequence which is obtained by frequency transformation of an input audio signal, and second-and-onward-stages of vector quantization processes for vector-quantizing a quantization error component in the previous-stage vector quantization process: wherein, among the multiple stages of quantization processes according to the multiple-stage quantization method, at least one vector quantization process performs vector quantization using, as weighting coefficients for quantization, weighting coefficients on frequency, calculated on the basis of the spectrum of the input audio signal and the auditive sensitivity characteristic showing the auditive nature of human beings; and, on the basis of the spectrum of the input audio signal and the auditive sensitivity characteristic showing the auditive nature of human beings, a frequency block having a high importance for quantization is selected from frequency blocks of the quantization error component in the first-stage vector quantization process and, in the second-stage vector quantization process, the quantization error component of the first-stage quantization process is quantized with respect to the selected frequency block.
Another audio signal coding apparatus according to the present invention comprises: a time-to-frequency transformation unit for transforming an input audio signal to a frequency-domain signal, a spectrum envelope calculation unit for calculating a spectrum envelope of the input audio signal; a normalization unit for normalizing the frequency-domain signal obtained in the time-to-frequency transformation unit, with the spectrum envelope obtained in the spectrum envelope calculation unit, thereby to obtain a residual signal; an auditive weighting calculation unit for calculating weighting coefficients on frequency, on the basis of the spectrum of the input audio signal and the auditive sensitivity characteristic showing the auditive nature of human beings; and a multiple-stage quantization unit having multiple stages of vector quantization units connected in columns, to which the normalized residual signal is input, at least one of the vector quantization units performing quantization using weighting coefficients obtained in the weighting unit.
Another audio signal coding apparatus according to the present invention includes plural quantization units among the multiple stages of the multiple-stage quantization unit that perform quantization using the weighting coefficients obtained in the weighting unit, and the auditive weighting calculation unit calculates individual weighting coefficients to be used by the multiple stages of quantization units, respectively.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the multiple-stage quantization unit comprises: a first-stage quantization unit for quantizing the residual signal normalized by the normalization unit, using the spectrum envelope obtained in the spectrum envelope calculation unit as weighting coefficients in the respective frequency domains; a second-stage quantization unit for quantizing a quantization error signal from the first-stage quantization unit, using weighting coefficients calculated on the basis of the correlation between the spectrum envelope and the quantization error signal of the first-stage quantization unit, as weighting coefficients in the respective frequency domains, and a third-stage quantization unit for quantizing a quantization error signal from the second-stage quantization unit using, as weighting coefficients in the respective frequency domains, weighting coefficients which are obtained by adjusting the weighting coefficients calculated by the auditive weighting calculating unit according to the input signal transformed to the frequency-domain signal by the time-to-frequency transformation unit and the auditive characteristic, on the basis of the spectrum envelope, the quantization error signal of the second-stage quantization unit, and the residual signal normalized by the normalization unit.
Another audio signal coding apparatus according to the present invention comprises: a time-to-frequency transformation unit for transforming an input audio signal to a frequency-domain signal, a spectrum envelope calculation unit for calculating a spectrum envelope of the input audio signal; a normalization unit for normalizing the frequency-domain signal obtained in the time-to-frequency transformation unit, with the spectrum envelope obtained in the spectrum envelope calculation unit, thereby to obtain a residual signal; a first vector quantizer for quantizing the residual signal normalized by the normalization unit; an auditive selection means for selecting a frequency block having a high importance for quantization among frequency blocks of the quantization error component of the first vector quantizer, on the basis of the spectrum of the input audio signal and the auditive sensitivity characteristic showing the auditive nature of human beings; and a second quantizer for quantizing the quantization error component of the first vector quantizer with respect to the frequency block selected by the auditive selection means.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the auditive selection means selects a frequency block using, as a scale of importance to be quantized, a value obtained by multiplying the quantization error component of the first vector quantizer, the spectrum envelope signal obtained in the spectrum envelope calculation unit, and an inverse characteristic of the minimum audible limit characteristic.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the auditive selection means selects a frequency block using, as a scale of importance to be quantized, a value obtained by multiplying the spectrum envelope signal obtained in the spectrum envelope calculation unit and an inverse characteristic of the minimum audible limit characteristic.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the auditive selection means selects a frequency block using, as a scale of importance to be quantized, a value obtained by multiplying the quantization error component of the first vector quantizer, the spectrum envelope signal obtained in the spectrum envelope calculation unit, and an inverse characteristic of a characteristic obtained by adding the minimum audible limit characteristic and a masking characteristic calculated from the input signal.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the auditive selection means selects a frequency block using, as a scale of importance to be quantized, a value obtained by multiplying the quantization error component of the first vector quantizer, the spectrum envelope signal obtained in the spectrum envelope calculation unit, and an inverse characteristic of a characteristic obtained by adding the minimum audible limit characteristic and a masking characteristic that is calculated from the input signal and corrected according to the residual signal normalized by the normalization unit, the spectrum envelope signal obtained in the spectrum envelope calculation unit, and the quantization error signal of the first-stage quantization unit.
An audio signal coding apparatus according to the present invention is an apparatus for coding a data quantity by vector quantization using a multiple-stage quantization means comprising a first vector quantizer for vector-quantizing a frequency characteristic signal sequence obtained by frequency transformation of an input audio signal, and a second vector quantizer for vector-quantizing a quantization error component of the first vector quantizer: wherein the multiple-stage quantization means divides the frequency characteristic signal sequence into coefficient streams corresponding to at least two frequency bands, and each of the vector quantizers performs quantization, independently, using a plurality of divided vector quantizers which are prepared corresponding to the respective coefficient streams.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus further comprising a normalization means for normalizing the frequency characteristic signal sequence.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the quantization means appropriately selects a frequency band having a large energy-addition-sum of the quantization error, from the frequency bands of the frequency characteristic signal sequence to be quantized, and then quantizes the selected band.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the quantization means appropriately selects a frequency band from the frequency bands of the frequency characteristic signal sequence to be quantized, on the basis of the auditive sensitivity characteristic showing the auditive nature of human beings, which frequency band selected has a large energy-addition-sum of the quantization error weighted by giving a large value to a band having a high importance of the auditive sensitivity characteristic, and then the quantization means quantizes the selected band.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the quantization means has a vector quantizer serving as an entire band quantization unit which quantizes, once at least, all of the frequency bands of the frequency characteristic signal sequence to be quantized.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the quantization means is constructed so that the first-stage vector quantizer calculates a quantization error in vector quantization using a vector quantization method with a code book and, further, the second-stage quantizer vector-quantizes the calculated quantization error.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein, as the vector quantization method, code vectors, all or a portion of which codes are inverted, are used for code retrieval.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus further comprising a normalization means for normalizing the frequency characteristic signal sequence, wherein calculation of distances used for retrieval of an optimum code in vector quantization is performed by calculating distances using, as weights, normalized components of the input signal processed by the normalization unit, and extracting a code having a minimum distance.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the distances are calculated using, as weights, both of the normalized components of the frequency characteristic signal sequence processed by the normalization means and a value in view of the auditive sensitivity characteristic showing the auditive nature of human beings, and a code having a minimum distance is extracted.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the normalization means has a frequency outline normalization unit that roughly normalizes the outline of the frequency characteristic signal sequence.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the normalization means has a band amplitude normalization unit that divides the frequency characteristic signal sequence into a plurality of components of continuous unit bands, and normalizes the signal sequence by dividing each unit band with a single value.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the quantization means includes a vector quantizer for quantizing the respective coefficient streams of the frequency characteristic signal sequence independently by divided vector quantizers, and includes a vector quantizer serving as an entire band quantization unit that quantizes, once at least, all of the frequency bands of the input signal to be quantized.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the quantization means comprises a first vector quantizer comprising a low-band divided vector quantizer, an intermediate-band divided vector quantizer, and a high-band divided vector quantizer, and a second vector quantizer connected after the first quantizer, and a third vector quantizer connected after the second quantizer. The frequency characteristic signal sequence input to the quantization means is divided into three bands, and the frequency characteristic signal sequence of low-band component among the three bands is quantized, by the low-band divided vector quantizer. The frequency characteristic signal sequence of intermediate-band component among the three bands is quantized by the intermediate-band divided vector quantizer, and the frequency characteristic signal sequence of high-band component among the three bands is quantized by the high-band divided vector quantizer, independently. A quantization error with respect to the frequency characteristic signal sequence is calculated in each of the divided vector quantizers constituting the first vector quantizer, and the quantization error is input to the subsequent second vector quantizer. The second vector quantizer performs quantization for a band width to be quantized by the second vector quantizer, calculates a quantization error with respect to the input of the second vector quantizer, and inputs this to the third vector quantizer. The third vector quantizer performs quantization for a band width to be quantized by the third vector quantizer.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus further comprising a first quantization band selection unit between the first vector quantizer and the second vector quantizer, and a second quantization band selection unit between the second vector quantizer and the third vector quantizer: wherein the output from the first vector quantizer is input to the first quantization band selection unit, and a band to be quantized by the second vector quantizer is selected in the first quantization band selection unit. The second vector quantizer performs quantization for a band width to be quantized by the second vector quantizer, with respect to the quantization errors of the first three vector quantizers decided by the first quantization band selection unit, calculates a quantization error with respect to the input to the second vector quantizer, and inputs this to the second quantization band selection unit. The second quantization band selection unit selects a band to be quantized by the third vector quantizer. The third vector quantizer performs quantization for a band decided by the second quantization band selection unit.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein, in place of the first vector quantizer, the second vector quantizer or the third vector quantizer is constructed using the low-band divided vector quantizer, the intermediate-band divided vector quantizer, and the high-band divided vector quantizer.
Another audio signal decoding apparatus according to the present invention is an apparatus receiving, as an input, codes output from the audio signal coding apparatus and decoding these codes to output a signal corresponding to the original input audio signal, and this apparatus comprises: an inverse quantization unit for performing inverse quantization using at least a portion of the codes output from the quantization means of the audio signal coding apparatus and an inverse frequency transformation unit for transforming a frequency characteristic signal sequence output from the inverse quantization unit to a signal corresponding to the original audio input signal.
Another audio signal decoding apparatus according to the present invention is an apparatus receiving, as an input, codes output from the audio signal coding apparatus and decoding these codes to output a signal corresponding to the original input audio signal, and this apparatus comprises: an inverse quantization unit for reproducing a frequency characteristic signal sequence; an inverse normalization unit for reproducing normalized components on the basis of the codes output from the audio signal coding apparatus, using the frequency characteristic signal sequence output from the inverse quantization unit, and multiplying the frequency characteristic signal sequence and the normalized components; and an inverse frequency transformation unit for receiving the output from the inverse normalization unit and transforming the frequency characteristic signal sequence to a signal corresponding to the original audio signal.
Another audio signal decoding apparatus according to the present invention is an apparatus receiving, as an input, codes output from the audio signal coding apparatus and decoding these codes to output a signal corresponding to the original audio signal, and this apparatus comprises an inverse quantization unit which performs inverse quantization using the output codes whether the codes are output from all of the vector quantizers constituting the quantization means in the audio signal coding apparatus or from some of them.
Another audio signal decoding apparatus according to the present invention is an audio signal decoding apparatus wherein the inverse quantization unit performs inverse quantization of quantized codes in a prescribed band by executing, alternately, inverse quantization of quantized codes in a next stage, and inverse quantization of quantized codes in a band different from the prescribed band. When there are no quantized codes in the next stage during the inverse quantization, the inverse quantization unit continuously executes the inverse quantization of quantized codes in the different band and, when there are no quantized codes in the different band, the inverse quantization unit continuously executes the inverse quantization of quantized codes in the next stage.
Another audio signal decoding apparatus according to the present invention is an apparatus receiving, as an input, codes output from the audio signal coding apparatus and decoding these codes to output a signal corresponding to the original input audio signal, and this apparatus comprises an inverse quantization unit which performs inverse quantization using only codes output from the low-band divided vector quantizer as a constituent of the first vector quantizer even though all or some of the three divided vector quantizers constituting the first vector quantizer in the audio signal coding apparatus output codes.
Another audio signal decoding apparatus according to the present invent ion is an audio signal decoding apparatus wherein the inverse quantization unit performs inverse quantization using codes output from the second vector quantizer, in addition to the codes output from the low-band divided vector quantizer as a constituent of the first vector quantizer.
Another audio signal decoding apparatus according to the present invention is an audio signal decoding apparatus wherein the inverse quantization unit performs inverse quantization using codes output from the intermediate-band divided vector quantizer as a constituent of the first vector quantizer, in addition to the codes output from the low-band divided vector quantizer as a constituent of the first vector quantizer and the codes output from the second vector quantizer.
Another audio signal decoding apparatus according to the present invention is an audio signal decoding apparatus wherein the inverse quantization unit performs inverse quantization using codes output from the third vector quantizer, in addition to the codes output from the low-band divided vector quantizer as a constituent of the first vector quantizer, the codes output from the second vector quantizer, and the codes output from the intermediate-band divided vector quantizer as a constituent of the first vector quantizer.
Another audio signal decoding apparatus according to the present invention is an audio signal decoding apparatus wherein the inverse quantization unit performs inverse quantization using codes output from the high-band divided vector quantizer as a constituent of the first vector quantizer, in addition to the codes output from the low-band divided vector quantizer as a constituent of the first vector quantizer, the codes output from the second vector quantizer, the codes output from the intermediate-band divided vector quantizer as a constituent of the first vector quantizer, and the codes output from the third vector quantizer.
Another audio signal coding apparatus according to the present invention comprises: a phase information extraction unit for receiving, as an input signal, a frequency characteristic signal sequence by obtained by frequency transformation of an input audio signal, and extracting phase information of a portion of the frequency characteristic signal sequence corresponding to a prescribed frequency band, a code book for containing a plurality of audio codes being representative values of the frequency characteristic signal sequence, wherein an element portion of each audio code corresponding to the extracted phase information is shown by an absolute value; and an audio code selection unit for calculating the auditive distances between the frequency characteristic signal sequence and the respective audio codes in the code book, selecting an audio code having a minimum distance, adding phase information to the audio code having the minimum distance using the output from the phase information extraction unit as auxiliary information, and outputting a code index corresponding to the audio code having the minimum distance as an output signal.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein the phase information extraction unit extracts phase information of a prescribed number of elements on the low-frequency band side of the input frequency characteristic signal sequence.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus further comprising an auditive psychological weight vector table which is a table of auditive psychological quantities relative to the respective frequencies in view of the auditive psychological characteristic of human beings: wherein the phase information extraction unit extracts phase information of an element which matches with a vector stored in the auditive psychological weight vector table, from the input frequency characteristic signal sequence.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus further comprising a smoothing unit for smoothing the frequency characteristic signal sequence using a smoothing vector by division between vector elements: wherein, before selecting the audio code having the minimum distance and adding the phase information to the selected audio code, the audio code selecting unit converts the selected audio code to an audio code which has not been subjected to smoothing using smoothing information output from the smoothing unit, and outputs a code index corresponding to the audio code as an output signal.
An audio signal coding apparatus according to the present invention is an audio signal coding apparatus further comprising: an auditive psychological weight vector table which is a table of auditive psychological quantities relative to the respective frequencies, in view of the auditive psychological characteristic of human beings; a smoothing unit for smoothing the frequency characteristic signal sequence using a smoothing vector by division between vector elements; and a sorting unit for selecting a plurality of values obtained by multiplying the values of the auditive psychological weight vector table and the values of the smoothing vector table, in order of auditive importance, and outputting these values toward the audio code selection unit.
Another audio signal coding apparatus according to the present invention is an audio signal-coding apparatus wherein employed as the frequency characteristic signal sequence is a vector of which elements are coefficients obtained by subjecting the audio signal to frequency transformation.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein employed as the frequency characteristic signal sequence is a vector of which elements are coefficients obtained by subjecting the audio signal to frequency transformation.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein employed as the frequency characteristic signal sequence is a vector of which elements are coefficients obtained by subjecting the audio signal to frequency transformation.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein employed as the frequency characteristic signal sequence is a vector of which elements are coefficients obtained by subjecting the audio signal to MDCT (Modified Discrete Cosine Transformation).
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein employed as the frequency characteristic signal sequence is a vector of which elements are coefficients obtained by subjecting the audio signal to MDCT (Modified Discrete Cosine Transformation).
Another audio signal coding, apparatus according to the present invention is an audio signal coding apparatus wherein employed as the frequency characteristic signal sequence is a vector of which elements are coefficients obtained by subjecting the audio signal to MDCT (Modified Discrete Cosine Transformation).
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein employed as the smoothing vector is a vector of which elements are relative frequency responses in the respective frequencies, which are calculated from linear prediction coefficients obtained by subjecting the audio signal to linear prediction.
Another audio signal coding apparatus according to the present invention is an audio signal coding apparatus wherein employed as the smoothing vector is a vector of which elements are relative frequency responses in the respective frequencies, which are calculated from linear prediction coefficients obtained by subjecting the audio signal to linear prediction.
Another audio signal decoding apparatus according to the present invention comprises: a phase information extraction unit for receiving, as an input signal, one of code indices obtained by quantizing frequency characteristic signal sequences which are feature quantities of an audio signal, and extracting phase information of elements of the input code index corresponding to a prescribed frequency band; a code book for containing a plurality of frequency characteristic signal sequences corresponding to the code indices, wherein an element portion corresponding to the extracted phase information is shown by an absolute value, and an audio code selection unit for calculating the auditive distances between the input code index and the respective frequency characteristic signal sequences in the code book, selecting a frequency characteristic signal sequence having a minimum distance, adding phase information to the frequency characteristic signal sequence having the minimum distance using the output from the phase information extraction unit as auxiliary information, and outputting the frequency characteristic signal sequence corresponding to the input code index as an output signal.